Fabrication method of functionally-graded structures by continuous ceramic filaments

ABSTRACT

A method for constructing a plurality of ceramic layers by winding continuous ceramic filaments to prepare RF-transparent structures is provided. Dielectric properties of each layer of the plurality of ceramic layers are characterized by an inter-filament spacing, a filament count and thickness. Once the plurality of ceramic layers are constructed, a structure is removed from a winding surface, wherein the winding surface is a mandrel, infiltrated with a resin in a separate set up and fired.

CROSS REFERENCE TO THE RELATED APPLICATIONS

This application is the national stage entry of InternationalApplication No. PCT/TR2019/050780, filed on Sep. 20, 2019, the entirecontents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention is a method by which multiple ceramic layers areconstructed by winding continuous ceramic filaments to prepareRF-transparent structures.

BACKGROUND

Advanced radar systems in hypersonic missiles impact the materials andthe production techniques used in traditional radome technology. Theneed to detect multiple targets effectively and faster by the radarwhilst withstanding the elevated temperatures, thermomechanical loadsand hostile environmental factors challenge the development of high-endmissile radomes.

Fiber-reinforced ceramic matrix composite (FR-CMC) is a promisingsolution to address most of the aforementioned concerns. Thesecomposites are manufactured by preparing 2D (woven, weft knitted,braided, warp knitted) or 3D (3D woven, 3D spacer) fabrics using ceramicfilaments such as fibers [1, 2], which are then impregnated by a ceramicsuspension. The ceramic fiber can be oxide or non-oxide depending on theapplication [3-5].

CMC technology to develop missile radomes have gained a significantmomentum in the last decades. U.S. Pat. No. 5,738,750 explains themethod to develop multilayer radome layers in which a honeycombstructure is covered with piles of quartz cloth that is composed ofsilica fiber (65 wt. %) infiltrated by silica-based resin (35 wt. %) onboth sides of the honeycomb. The inorganic resin is either polysiliconeor polysilozane, which is converted to silica or silicon nitride afterpyrolysis, respectively. However, a clear description of how the radomeshape is formed by joining these layers is not clearly mentioned.

In U.S. Pat. No. 7,118,802, the requirements for a missile radome flyingat 6+ Mach is disclosed. The proposed structure is composed of a loadbearing layer of colloid-impregnated FR-CMC and a thermal insulationlayer. The colloid is a ceramic suspension with 40-50 wt. % solidsloading (alumina or silica), while the insulation layer is a foam with45% opening filled with ceramic particles. The layers are bonded with ahigh temperature stable adhesive. Similar to the previously-mentionedpatent, this document also lacks a clear description of how the radomeis shaped by using these individual layers.

The construction of the broadband HARM anti-radiation missile issketched in [7]. According to this model, 3 mm thick, low dielectrichoneycomb structure is sandwiched between the thinner, high dielectriclayers. Similar to the disclosed information in open literature, thereis no explanation as to how the broadband radome is constructed.

Fabrication of ceramic broadband missile radomes impose severalrestrictions on the selection of materials and production technologies.Although the materials for super/hypersonic missile radomes arewell-known for decades, it is relatively recent to adopt the high-endtechnologies to develop broadband radomes flying at high Mach numbers.Consequently, there is limited information on fabrication of ceramicbroadband radomes, which are most likely prepared either by functionalgrading or by sandwich structures fulfilling the broadbandcharacteristic.

Previous efforts mostly focus on shaping big, one-layer ceramic radomesoperating at narrow/single band. Molding combined with tooling (U.S.Pat. No. 2002/0163109), cold isostatic pressing of cylinders andmachining (U.S. Pat. Nos. 9,673,518; 4,615,859; 4,615,933), slip casting[8], slip casting followed by chemical vapour deposition (CVD) (U.S.Pat. No. 4,358,772), additive manufacturing (U.S. Pat. No. 2009/0096687)are some of the techniques mentioned in literature.

Based on this information, the factors which impede the progress infabrication of ceramic broadband radomes can be summarized as follows:

-   -   Fragile nature of the ceramic material making it difficult to        shape and sinter for bigger pieces without defects.    -   CTE (Coefficient of Thermal Expansion) mismatch between the        individual ceramic layers of the multilayer structure leading to        micro cracks and delamination during firing.    -   Difficulty of grading the dielectric constant by porosity due to        the breakup of the structure along the pore chain.

SUMMARY

The present invention is a method by which multiple ceramic layers areconstructed by winding continuous ceramic filaments to prepareRF-transparent structures. The dielectric properties of each layer arecharacterized by the inter-filament spacing and the filament count. Oncethe multiple layers are constructed, the structure is removed from thewinding surface (e.g. mandrel), infiltrated with resin in a separate setup and fired.

Fabrication of the ceramic broadband missile radomes by ceramic fibernetworks as discussed in this invention has the following distinctivefeatures:

-   -   Intrinsically fragile ceramic material is shaped on a mandrel        with continuous ceramic filaments, which are bendable and        flexible.    -   The filaments can be selected from fibers, fiber bundles and        fabrics.    -   The filaments (termed as fibers from this point on) can be        selected from a range of oxide or non-oxide ceramics such as        SiO₂, Al₂O₃, SiC or the mixed compositions thereof.    -   The fibers can be of pure ceramic, organic vehicle added or PDC        (Polymer Derived Ceramic) origin, which are converted to pure        ceramic after debinding and firing.    -   Fibers of organic and synthetic nature (cotton, Aramid, Kevlar,        polyacrylonitrile and similar) can also be used as sacrificial        layers forming porosity (low dielectric regions) in the        structure up on firing.    -   The fibers can be wound, wrapped or braided on a support such as        mandrel in x, y, and z directions (processes termed as braiding        from this point on).    -   Each layer of the structure is formed by braiding the continuous        ceramic fiber with specific dielectric constant according to a        pattern. The winding angle on the mold, the aperture between the        fibers (layer porosity) and the winding count (layer thickness)        are the defining parameters of the pattern and hence, the        dielectric characteristics of the layer. In other words, the        dielectric layer is not necessarily defined by the material        itself, but the fiber network design. Such flexibility in        materials selection and in layer arrangement amplifies the RF        design capabilities.    -   The dielectric constant grading through porosity grading across        the thickness is determined by the ceramic fiber network        density; mainly by the inter-fiber aperture. Lower dielectric        constant layer is obtained by keeping the inter-fiber spacing        larger.    -   The porosity grading is not obtained by making the matrix        element gradually porous by using hard-to-control pore-formers        during firing but by the density of the ceramic fiber network.    -   Braided layers on the mold are removed from the mold as a basket        and impregnated with a resin of defined composition under vacuum        or pressure yielding a near net shape structure in the green        state already. This permits green-machining of the ceramic,        which accelerates the typical machining times for sintered        bodies.    -   Only one type of slurry (resin) is used, which forms the matrix.        Therefore, there is no concern of resin incompatibility between        the layers.    -   The use of one resin in all layers eliminates the risk of CTE        mismatch-related defects, since it represents one matrix of        homogenous composition across the layers.    -   The resin composition can be pure ceramic or inorganic-based,        which is converted to ceramic up on sintering through oxidation        or pyrolysis.    -   The structure is much tougher due to the inorganic resin that        fills the inter-fiber space. This composite structure helps the        structure fail gradually under operational conditions instead of        sudden and catastrophic fracture as in pure ceramic body.    -   The final structure is near net shape, which avoids the complex        and time taking processes leading to low productivity by        conventional techniques.

BRIEF DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows building the functionally-graded (dielectric-graded) layersover the mandrel using the identical fiber with different braidingdensity (inter-fiber aperture) and braiding count (layer thickness) oneach layer.

FIG. 2 shows building the functionally-graded layers over the mandrelusing the same fabric with different wrap density and wrap count on eachlayer.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The fiber-reinforced ceramic matrix composites (FR-CMC) are advanced andtailorable materials with improved toughness and damage tolerancecompared to bulk ceramics [6]. Broadly speaking, the reinforcing fiberscan be classified as inorganic and organic fibers [4]. The inorganicfibers can be further splitted to non-metallic and metallic fibers,while the organic fibers are mostly carbon and polymer fibers. Theceramic fibers belong to the non-metallic inorganic fibers familytogether with the glass/mineral and single crystal fibers [4].

The fiber material selection for CMC application is of paramountimportance. It is known that the temperatures on radome material duringhigh Mach number flights reach up to 1.000° C. limiting the choice ofthe fiber material. Polymer and glass fibers have 500° C. and 700° C. ofdegradation temperatures, respectively, which restrict their effectiveuse in CMCs at higher temperatures [6]. Therefore, the ceramic fiberscome out as the right choice to support the high performance CMCs forairborne components exposed to elevated temperatures andmechanical/thermomechanical loads at higher speeds.

The ceramic fibers are classified as oxide or non-oxide ceramics. Theones in the former group are alumina (Al₂O₃) based fibers exhibitinghigh environmental stability but limited high temperature creepperformance. The alumina composition of such fibers can be selected in arange from 10% to 100%. The non-oxide ceramic fibers are mostly SiC,which have excellent thermal creep behavior coupled with poor chemicalstability. SiC portion of these fibers can vary in the 10% to 100% rangedepending on the operational specifications. For both fiber classes,crystallinity, morphology, uniformity along the material and the surfaceproperties are important characteristics impacting the CMC performancein the field. Fiber coating is another critical factor determining thedamage tolerance of the structure by providing a weak interface betweenthe fiber and the matrix [4, 6]. The selection between the two fibertypes strongly depends on the type of the matrix or the inorganic resinfilling the fiber network. The oxide fibers should ideally be used withthe oxide matrix (oxide composite) and the non-oxide with the non-oxidematrix (non-oxide composite). However, intermediate mixtures are alsoprepared by different processing techniques, which lead to newerapplications.

As for oxide composites, the fibers prepared with pure Al₂O₃ or Al₂O₃blended with SiO₂ and B₂O₃ at lower concentrations significantlyincrease the oxidation and the alkaline resistance of CMC [3,4]. Fornon-oxide composites, SiC fibers coated with C or BN allow the SiCmatrix composite resist high temperature deformation [4]. The comparisonbetween the fiber and the bulk forms of the Al₂O₃ and SiC ceramics arepresented in Table 1. The significantly superior tensile strength of thefiber over the bulk is worth to mention for consideration of thesefibers under severe environmental conditions.

TABLE 1 Comparison of ceramic fiber vs. bulk ceramic properties MaterialAl₂O₃ SiC Properties Unit Fiber^(a) Bulk^(b) Fiber^(c) Bulk^(d) Densityg/cm³ 3.90 3.90 3.10 3.20 Tensile Strength MPa 2.930 400 2.600 540Elastic Modulus GPa 373 380 420 430 CTE (40-800° C.) ppm/° C. 8.00 8.003.00-3.50 3.70-4.40 Continuous Use ° C. 1.000* ~1.000⁺ 1.150** ~1.000⁺Temperature ^(a)Nextel 610, ^(b)Kyocera A601D (>99%) ^(c)Nippon CarbonHi-Nicalon “S” (99.8%), ^(d)Kyocera SC211 *Single filament ≤1% strain/69MPa/1.000 hr **Single filament 500 MPa/1.000 hr ⁺estimated

To sum up, the ceramic fibers provide toughness while improving thedamage tolerance of the bulk ceramics. The super/hypersonic missileradomes produced as bulk ceramics from materials such as fused silica,Magnesium Aluminum Silicate, Lithium Aluminum Silicate, Si₃N₄, SiAlON,Al₂O₃ run the risk of catastrophic failure under extreme conditions dueto their fragile nature. The techniques used in production of theseceramics such as slip casting, glass melt casting, hot molding have lowyields due to the fracture of the ceramic during consolidation, drying,firing and machining steps.

The focus of the presented method by which the ceramic fiber-reinforcedCMCs are prepared. By following this method, the ceramic fibers and theinorganic resins compatible with these fibers can be used to prepare theairborne structures such as radomes, microwave-transparent shields, capsand noses for military and civil applications flying at subsonic,supersonic and hypersonic velocities. There is no restriction incombination of available fibers and resins as long as the materialscompatibility and the RF-transparency at desired frequencies arefulfilled. Moreover, the method is applicable to build both broad,narrow and single band radomes. The type and the diameter of the fiber,braiding type, fiber aperture and thickness per layer, slurry materialcomposition are engineered for the desired electromagnetic performance.

In this invention, the continuous and identical ceramic fibers are usedto form the multiple layers of the broadband radome. Each layer isidentified by a specific fiber pattern, which is characterized by thewinding/braiding angle, braiding density (inter-fiber aperture) andwrapping count (layer thickness). Therefore, the pattern determines thedielectric characteristic of the layer through its inter-fiber apertureand the fiber thickness. The broadband characteristic of the radome canbe optimized by changing the layer characteristics.

The fabrication of the structure with graded porosity starts by braidingthe continuous ceramic fiber directly on a specific support surface suchas mandrel in a specific pattern to achieve the minimum dielectricconstant (maximum porosity) first. Prior to braiding, the mandrel iscoated with a non-sticking chemical to facilitate the easy removal ofthe braided structure at the end of the process. Once the desiredthickness of the first layer is wound, the next layers with increasingfiber density are braided one over another. Significant improvement canbe achieved in the structural integrity if the ceramic fibers atconsecutive layers are wrapped in an angular orientation between15°-135°. In this design, the mechanically weakest layer representingthe minimum dielectric constant is restricted to the innermost part ofthe radome and hence, protected from the hostile environment on theouter skin. This approach is represented in FIG. 1, where the mandrel iswrapped with 3 different layers, each specified by a unique pattern anddielectric constant. The 1st layer is of maximum fiber apertureexhibiting the minimum dielectric constant, ε₁. The fiber densityincreases gradually from 2nd through 3rd layers, giving dielectricconstants ε₂ and ε₃, respectively. It is important to mention that theε₁₋₃ are the dielectric constant values of the layers defined byspecific patterns of the same fiber and not the dielectric constant ofdifferent fibers. Once the 3 layers are braided, the multilayerstructure with graded porosity is removed from the mandrel and ready forthe infiltration process. The layer thickness is defined by the braidingcount and each layer thickness can be kept identical or alteredaccording to a specific RF design.

Alternatively, the ceramic fabrics can also be used to constructmultilayer and functionally ceramic structures as an alternative to thefibers. The fabrics are wider than fibers and hence, they accelerate thefabrication process. Should the fabrics replace the fibers, thestructure is constructed by processes similar to the aforementionedroute (FIG. 2). In this case, the mechanically-weaker layer with higherinter-fabric opening is braided first, as the innermost layer far fromthe outer surface (skin) that is prone to more aggressive conditions.Layer 1 (L_1) has the maximum fabric opening and hence the minimumdielectric constant, whereas layer 3 (L_3) exhibits the minimum fabricopening and hence, the maximum dielectric constant. Therefore, the orderof the dielectric constants of the layers can be written asε_(L_1)<ε_(L_2)<ε_(L_3).

Once the continuous fibers are wrapped on the mandrel and all layers ofthe structure satisfying the desired broadband performance are piled up,the structure is removed from the mandrel. It is basically a basketformed by an intense fiber network braided according to a specificdesign, which is ready for infiltration. The slurry infiltration is theprocess during which the slurry fills the inter-fiber gaps. This processcan best be conducted under vacuum, where the fiber basket is placed ina special chamber filled with the slurry.

Alternatively, the basket can be placed between and supported by femaleand male molds made of stainless steel with non-sticking surfaces, whichare fed with the slurry. In both methods, the vacuum is applied in theclosed chamber or molds, which moves the slurry with optimized rheologyinto the open space between the fibers.

In a different approach, the basket can be dipped in a container full ofthick slurry. The structure is then exposed to vacuum from the oppositeside without slurry (inner side), which pulls the slurry into theapertures between the fibers.

In all of these methods, the integrity of the fiber structure must beobserved carefully and preserved intact against a possible deformationcaused by vacuum. As further processing, machining of the firedstructures can also be considered and applied with no detrimentaleffects on the structure as the fibers follow the contour defined by thematrix.

The slurry infiltrated fiber network is dried and debinded cautiously.Since all thermal process have the potential to generate irreversibleimpacts on the structure such as crack initiation and propagation,fracture, sagging, bulging, collapsing, the debinding and sinteringprofiles must be carefully optimized. Therefore, the raw materials mustbe carefully characterized in terms of their compositions andrheological and thermo-mechanical behaviour prior to processing.

The described invention is applicable for continuous oxide/non-oxidefibers and the slurries compatible with these fibers. In other words,the fiber-slurry pair has to be defined together to guarantee thematerials' compatibility and the performance of the final structure. Thefiber should have a sintering temperature comparable to the temperaturestability range of the matrix, low CTE, low dielectric constant and lossand high thermo-stability and mechanical strength. Moreover, thesecharacteristics are expected to be preserved/slightly deviate withtemperature fluctuations. Most of these requirements are well satisfiedby fused silica, which is used in commercial missile radomes fordecades. Therefore, PDC-based slurries with polysilicone, polysilozane,polycarbosilane are candidate slurries to use with selected fibers.Alternatively, slurries with materials such as alumina at varyingcompositions can also be used as long as the aforementioned fiber-slurryspecs are matched.

The fiber selection for current radome materials such as fused silica,Magnesium Aluminum Silicate, Lithium Aluminum Silicate, Si₃N₄, SiAlON,Al₂O₃ is limited. Among all commercial products, Al₂O₃ and SiC are thecommercially-available candidates for oxide and non-oxide fibers,respectively. The former is produced in different compositions toaddress the requirements in diverse applications, whilst the latter isnot fully appropriate as a radome material due to its reportedsemi-conductive character at high temperatures. The disclosed inventionovercomes this limitation through the arrangement of the dielectriclayers of the broadband structure by fiber design and not by thematerial itself. The wrapping density (inter-fiber aperture) and thewrap count (layer thickness) are the two major parameters defining thedielectric constant of each layer.

REFERENCES

1 B. Kumar, J. Hu, Woven Fabric Structures and Properties, Engineeringof High-Performance Textiles, 2018, 133-151.

2 K. Bilisik, N. Sahbaz Karaduman, N. E. Bilisik, 3D Fabrics forTechnical Textile Applications, INTECH, 2016, 81-141.

3 B. Klauss, B. Schawallar, Modern Aspects of Ceramic Fiber Development,2006, Advances in Science and Technology, Vol. 50, 1-8.

4 B. Clauss, Fibers for Ceramic Matrix Composites, Chapter 1, CeramicMatrix Composites. Edited by Walter Krenkel, WILEY-VCH Verlag GmbH & Co.KGaA, 2008, 1-20.

5 Nextel Application Brochure, 1-16.

6 Ceramic Fibers and Coatings, National Academy Press Washington, D.C.1998.

7 U.S.A.F. Avionics Laboratory, Development of Lightweight BroadbandRadomes From Slip-Cast Fused Silica”, 1966.

8 D. C. Chang, Comparison of Computed and Measured Transmission Data forthe AGM-88 HARM Radome, 1993, MSc Thesis, Naval Postgraduate School.

What is claimed is:
 1. A method for fabricating functionally-gradedstructures by continuous ceramic fibers, comprising the following stepsof: braiding the continuous ceramic fibers directly on a support surfacein a specific pattern, wherein a winding angle, a direction, adensity/inter-fiber aperture and a count/thickness for each layer oflayers are altered for other layers of a multi-layer design to fulfilldesired mechanical, thermal and electrical requirements. wrapping thecontinuous ceramic fibers at consecutive layers in an angularorientation between 15°-135° for optimizing a structural integrity andan RF performance, removing a structure from the support surface, andapplying a slurry infiltration over the structure to fill inter-fibergaps.
 2. The method according to claim 1, wherein the support surface iscoated with a non-sticking chemical prior to the step of braiding tofacilitate a removal of a braided structure.
 3. The method according toclaim 1, wherein the step of applying the slurry infiltration isconducted under a vacuum, wherein the vacuum is moving a slurry with anoptimized rheology into open spaces between the continuous ceramicfibers.
 4. The method according to claim 3, wherein the structure isplaced in a chamber/container filled with the slurry or placed betweenand supported by female and male molds made of a stainless steel withnon-sticking surfaces, wherein the female and male molds are fed withthe slurry under pressure.
 5. The method according to claim 1, whereindielectric constants of the layers are gradually increased or decreaseddepending on braiding patterns of an identical fiber.
 6. (canceled) 7.The method according to claim 1, wherein the structure is applicable tobuild broadband, narrow and single band missile radomes.
 8. The methodaccording to claim 1, wherein a finished structure is net-shaped,minimizing post-processing times and related product damages or relatedproduct losses.